Estimates of future high, median, and low ground- water levels are needed for engineering and architectural design decisions and for appropriate selection of land uses. For example, the failure of individual underground sewage-disposal systems due to high ground-water levels can be prevented if accurate water-level estimates are available. Estimates of extreme or average conditions are needed because short duration preconstruction obser- vations are unlikely to be adequately represen- tative. Water-level records for 40 U.S. Geological Survey observation wells in Rhode Island were used to describe and interpret water-level fluctuations. The maximum annual range of water levels average about 6 feet in sand and gravel and 11 feet in till. These data were used to develop equations for estimating future high, median, and low water levels on the basis of any one measurement at a site and records of water levels at observation wells used as indexes. The estimating technique relies on several assumptions about temporal and spatial variations: (1) Water levels will vary in the future as they have in the past, (2) Water levels fluctuate seasonally (3) Ground-water fluctuations are dependent on site geology, and (4) Water levels throughout Rhode Island are subject to similar precipitation and climate. Comparison of 6,697 estimates of high, median, and low water levels (depth to water level exceeded 95, 50, and 5 percent of the time, respectively) with the actual measured levels exceeded 95, 50, and 5 percent of the time at 14 sites unaffected by pumping and unknown reasons, yielded mean squared errors ranging from 0.34 to 1.53 square feet, 0.30 to 1.22 square feet, and 0.32 to 2.55 square feet, respectively. (USGS)
